Renal infections radiology

RENAL INFECTIONS

DR ASHISH GUPTA PG RADIODIAGNOSIS

SLIMS

Introduction Urinary tract infections are the most

common urologic disease. Diagnosis is based on typical clinical

symptoms and laboratory findings. In general, imaging is not necessary for

diagnosis and treatment of uncomplicated urinary tract infections in adult patients.

However, diagnostic imaging demonstrates the extent and nature of the urologic infections and their potential complications.

Urinary tract infection typically originates in the urinary bladder;

when it migrates to the kidney or is seeded there hematogenously,

a tubulointerstitial inflammatory reaction ensues, involving the renal pelvis and parenchyma.

The condition is characterized as PYELONEPHRITIS

Risk factors for upper urinary tract infection

Immune suppression (AIDS, diabetes, corticosteroidtherapy, chemotherapy to treat cancer, kidneytransplant) Recent treatment with antibiotics for urothelial

saprophyte bacterial flora imbalance, Urinary catheter. Pregnancy. Obstruction to the urinary collecting system: Urinary lithiasis, deformity to the urinary system, Urothelial neoplasm, bladder neuropathy

Upper Renal tract infection Associated with  renal pelvic, renal calyceal and 

renal parenchymal inflammation, and comprises a heterogeneous group of conditions.

bacterial pyelonephritis chronic pyelonephritis renal tuberculosis emphysematous pyelitis emphysematous pyelonephritis malacoplakia fungal pyelonephritis xanthogranulomatous pyelonephritis (XGP)

Pyelitis

inflammation of the mucosa and the collecting system .

On sonography this appears as echogenic and even thickening around the circumference of the walls of the collecting system.

This thickening is also seen on CT and MRI scans, but sonography is usually sufficient to put forward this diagnosis

Bilateral pyelitis

Bilateral pyelitis: a: in the axial plane; b: in the sagittal plane. Even thickening and contrast uptake in the walls of the renalpelvis (white arrow). The presence of cortical defects is visible and this corresponds to the sequelae of pyelonephritis (arrow-head)

Acute bacterial pyelonephritis 

common and continues to have significant morbidity in certain patients groups.

Epidemiology The incidence of acute pyelonephritis

parallels that of lower urinary tract infections: approximately five times more common in females with a sharp increase following puberty .

Clinical presentation

Clinical presentation is fairly specific and classical in most cases, consisting of rapid onset of High Fevers , Flank Pain And Tenderness.

White cells and bacteria are usually present in the urine, and blood tests reveal the expected changes: increased WCC, CRP and/or ESR. In severe cases, systemic sepsis may be present.

In many instances patients respond promptly to antibiotics and no imaging is required.

Pathology

The most commonly implicated organisms are from the gastrointestinal tract :

E. coli (most common) Klebsiella sp. Proteus sp. Enterobacter sp. Pseudomonas sp. Haemophilus influenzae

Infection gains access to the upper urinary tract by passing retrogradely up the ureter from the bladder, facilitated by virulence factors which allow bacteria to adhere to the urothelium (e.g. adhesin P) and inhibit ureteric peristalsis (endotoxins) .

Infection then passes into the collecting tubules and results in an interstitial nephritis, with resulting alterations in renal filtration and blood flow in the affected region.

Localised ischaemia secondary to inflammatory changes results in altered imaging and potentially eventually results in necrosis and scarring .

Rarely, the kidney may be seeded haematogeneously, in which case usually peripherally located renal abscesses develop rather than pyelonephritis. 

Radiographic features

In many instances imaging is not required. Situations in which imaging is indicated include: Exclude obstructed kidney High risk patients: diabetics, elderly,

immunocompromised Those with mixed clinical picture Previous renal pathology

Plain film

Plain films have a limited role to play, especially if patients are likely to go onto CT.

They may demonstrate obstructing urinary tract calculi and occasionally demonstrate gas within the collecting system (emphysematous pyelonephritis).

Findings seen in cases of acute kidney infections include renal enlargement, striated or delayed nephrograms, delayed caliceal appearance time, and dilatation or effacement of the collecting system

intravenous pyelographyAcute bacterial pyelonephritis of the leftkidney. Tomogram from intravenous pyelographydemonstrates an enlarged left kidney with effacementof the central collecting system.

Ultrasound

Ultrasound is insensitive to the changes of acute pyelonephritis, with most patients having 'normal' scan, and abnormalities only identified in ~25% of cases .

Possible features include: particulate matter in the collecting system reduced areas of of cortical vascularity by using power Doppler  gas bubbles (emphysematous pyelonephritis) abnormal echogenicity of the renal parenchyma 

› focal/segmental hypoechoic regions› mass like change

Ultrasound is however useful in assessing for local complications such as hydronephrosis,renal abscess formation, renal infarction, perinephric collections, and thus guiding management.

Ultrasound

US shows a round shaped hyperechoic focus in the inferior pole of the rightkidney related to acute bacterial pyelonephritis. US also shows mild dilatation of thecollecting system in the right kidney.

US

(a) US scan shows a wedge-shaped hyperechoic focus (arrowhead)in the upper pole of the right kidney related to acute bacterial pyelonephritis. (b) Color flow US image demonstratesdiminished flow through the involved area.

US power doppler

Hipovascular area in the upper pole with power Doppler, due to acutepyelonephritis.

CT CT is the most sensitive modality for the renal tract, able to assess

for renal calculi, gas, perfusion defects, collections and obstruction Non-contrast CT Often the kidneys appear normal. Affected parts of the kidney

typically may appear swollen and of lower attenuation. Renal calculi or gas within the collecting system may be evident.

Post-contrast CT Following administration of contrast, one or more focal wedge like

regions will appear swollen and demonstrate reduced enhancement compared to the normal portions of the kidney.

The periphery of the cortex is also affected, helpful in distinguishing so called lobar nephronia from a renal infarct (which tends to spare the periphery; so-called rim sign).

If imaged during the excretory phase, a striated nephrogram may also be visible .

If for some reason the kidney is imaged again within 3-6 hours, persistent enhancement of the affected regions may be evident due to slow flow of contrast through involved tubules .

CT without contrast

CT without contrast material shows focal hyper-attenuation areas in the upper poleof the right kidney which does not present enhancement with contrast administration,findings suggestive of hemorrhagic acute bacterial pyelonephritis.

Contrast-enhanced CT

Right pyelonephritis ona contrast-enhanced CT scan of the abdomen and pelvis in the tubularvenous phase: the renal parenchyma has a ‘‘spoked wheel’’appearance.

Severe unilateral acute bacterial pyelonephritis.

(a) US image demonstrates a slightly enlarged right kidney that is otherwise unremarkable, belying the advanced disease. (b) CT scan shows the enlarged kidney with global decreased uptake of contrast material and multiple small low-attenuation foci from abscess pockets, findingsthat prompted nephrectomy.

Mass like appearance of acute bacterial pyelonephritis

(a) US scan demonstrates a geographic,slightly lobulated “mass” (arrowhead) in the midpole of the left kidney, a finding that is worrisome for a solid tumor. (b) CT scan shows multifocal regions of diminished enhancement that extend to the periphery of the kidney, findings consistent with interstitial nephritis.

MRI MRI is usually reserved for patients who are pregnant,

and findings mirror those seen on CT. The kidney demonstrates wedge shaped regions of altered signal:

T1: affected region(s) appear hypointense compared to normal kidney parenchyma

T2: hyperintense compared to normal kidney parenchyma

T1 C+: reduced enhancement A fast inversion recovery sequence obtained after

contrast administration has been shown to be particularly effective in outlining affected regions which appear hyperintense compared to the low signal parenchyma. The contrast is thought to represent a combination of local oedema, and decreased T2 signal due to Gadolinium in the perfused 'normal' portions .

MR imaging of acute bacterialpyelonephritis

Sagittal short inversiontimeinversion recovery image of the right kidney obtained after gadolinium administrationdemonstrates signal drop off in the normal middle and lower renal poles due to normal perfusion and uptake of contrast agent. The infected upper pole, with its compromised perfusion, remains bright (arrowhead) and stands out in relief.

Nuclear medicine

Technetium-99m di-mercapto-succinic acid (DMSA) demonstrates a similar reduction in renal perfusion and function, which one or more wedge like defects in the outline of the kidneys .

ScintiscanScintiscan obtained with technetium 99m di-mercapto-succinic acid demonstratesa photopenic, peripheral defect(arrow) in the upper lateral margin of theright kidney that correlates with an area ofacute bacterial pyelonephritis

Treatment and prognosis

Most patients respond rapidly to appropriate antibiotic therapy, and often no imaging whatsoever is required. Even when imaging has been obtained, unless patients do not respond clinically no follow-up imaging is usually required, and it is important to note that imaging changes can take up to five months to resolve .

Presence of an upper urinary tract infection in the presence of obstruction can threaten the viability of the kidney and a percutaneous nephrostomy is usually required on an emergency basis.

Complications

Complications include : renal abscess renal infarction, necrosis and scarring chronic renal impairment hypertension

Differential diagnosis

General imaging differential considerations include

renal infarction› typically spares the peripheral aspect of

the cortex, e.g. cortical rim sign other causes of interstitial nephritis

› sarcoidosis› drug induced

Chronic pyelonephritis A form of pyelonephritis where there are

longstanding sequelae of renal infection. When acute pyelonephritis occurs repeatedly,

usually in relation to occult vesico-ureteric reflux, this can lead to the patient developing fibrosing interstitial nephritis

Very often it progresses slowly into renal failure. May arises from multiple recurrent infections, or

represents stable changes from a remote single infection, as radiographic differentiation between the above scenarios can be challenging.

Radiographic features

General Imaging should be interpreted in the relevant clinical

context and is often characterised by 

renal scarring renal atrophy renal cortical thinning compensatory hypertrophy of residual normal tissue

(which may mimic a mass lesion) calyceal clubbing: secondary to retraction of the

papilla from overlying scar  thickening and dilatation of the calyceal system overall renal asymmetry

Chronic pyelonephritis On SONOGRAPHY, atrophied kidneys appear

small in size, with dedifferentiation of the renal cortex and medulla and the presence of cortical notches.

If the patient’s renal function permits, a CONTRAST-ENHANCED CT SCAN will lead to diagnosis. It demonstrates the pyelonephritis scar tissue, which combines cortical retraction with deformities of the calyces, with areas in between that are comparatively healthy.

If there is no specific affected area, a RETROGRADE CYSTOGRAPHY must be carried out to investigate whether there is vesico-ureteric reflux.

Chronic pyelonephritis

(a) Unenhanced CT scan shows a small, deformed right kidney with multiple deep scars and dystrophic calcifications. (b) Photograph of the resected kidneys demonstrates extensive bilateral scar formation.

Chronic pyelonephritis

Chronic pyelonephritis: a: axial view; b: coronal reconstruction. Pyelonephritis scar tissue combining cortical retraction (whitearrows) and deformation of the calyces with areas in between that are comparatively healthy seen on contrast-enhanced CT scan.

CT Chronic pyelonephritis

CT Chronic pyelonephritis

Complications

Hypertension is frequently a long-term sequela.

Practical points Once the imaging changes of chronic

pyelonephritis have been established, repeat imaging is seldom provides new findings.

Renal tuberculosis A subset of genitourinary tuberculosis,

accounts for 15-20% of extra-pulmonary tuberculosis and can result in varied and striking radiographic appearances. 

Tuberculosis can involve both the renal parenchyma and the collecting system (calyces, renal pelvis, ureter, bladder and urethra) and results in different clinical presentations and radiographic appearances.

Clinical presentation Clinical features are often non specific and

include: haematuria flank pain constitutional symptoms Diagnosis can be obtained by culturing

multiple first-morning-void urine samples, or by histology of imaging guided biopsy or surgical specimens, although as with tuberculosis infection elsewhere the diagnosis is not always easy to confirm 4. 

Pathology Renal infection results from haematogenous spread

at the time of primary infection, with multiple micro-abscesses developing at the site of periglomerular capillary seeding.

Normal host immunity is usually able to dampen the disease with the usual development of a small inactive granuloma. 

Usually there is a long latency between primary infection and presentation which in most case occurs due to host immunity becoming compromised.

These quiescent granulomas then can reactivate, grow and eventually communicate with the calyces, leading to downstream infection. 

Radiographic features

Both the renal parenchyma and the upper collecting system (calyces and renal pelvis) can be involved. The former is usually seen associated with the latter, which is the most commonly involved site in the genitourinary tract. 

Infection limited to the renal parenchyma has two morphological appearances :

pyelonephritis › appearances are similar to pyelonephritis caused by other

organisms› hypoperfusion and swelling of all or part of the kidney

pseudotumoural type› single or multiple nodules › mimics renal cell carcinoma

Radiographic features Usually the collecting system is involved (either in

isolation or in combination with the parenchyma), and appearances vary according to the stage of disease . 

early› papillary necrosis (single or multiple) resulting in uneven

caliectasis  progressive

› multifocal strictures and hydronephrosis› mural thickening and enhancement (on cross-sectional

imaging)  endstage

› progressive hydronephrosis and parenchymal thinning› dystrophic calcification

Plain film

Plain film findings focus on calcification, which is seen in 25-45%, at various stages of disease. 

triangular in papillary necrosis focal or amorphous: putty kidney

 (endstage)

Calcifications of tuberculosis

(a) Abdominal radiograph demonstrates extensive calcifications forming a cast of the kidney and ureter.(b) Photograph of the cut specimen shows complete replacement of the normal kidneyby inflammatory debris

Fluoroscopy: IVP Traditional plain film IVP is quite sensitive to renal

tuberculosis with only 10% of affected patients having normal imaging. Features include:

parenchymal scars 50% moth eaten calyces: early finding         irregular caliectasis           phantom calyx hydronephrosis Lower urinary tract signs (see bladder and ureteric

tuberculosis) also recognised include: Kerr kink  sawtooth ureter pipe-stem ureter beaded or corkscrew ureter thimble bladder

Tuberculosis Retrograde pyelogram

Collimated image from intravenous urography demonstratesmultiple papillary cavities.

Retrograde pyelogram shows that the upper pole calix is stenotic (arrow) with associated papillary necrosis. The adjacent calix is fibrotic and distorted as well.

Ultrasound Sonographic appearances are non-specific and variable, depending

on the stage of disease.  early

› normal kidney or small focal cortical lesions with poorly defined border › +/- calcification.

progressive› papillary destruction with echogenic masses near calyces› distorted renal parenchyma› irregular hypoechoic masses connecting to collecting system; no renal

pelvic dilatation› mucosal thickening +/- ureteric and bladder involvement› small, fibrotic thick-walled bladder› echogenic foci or calcification (granulomas) in bladder wall near ureteric

orifice› localised or generalised pyonephrosis

endstage› small, shrunken kidney, "paper-thin" cortex and dense dystrophic

calcification in collecting system.› may resemble chronic renal disease

Ultrasound

Ultrasound is less sensitive than CT in detection of:

calyceal, pelvic or ureteral abnormalities.

isoechoic parenchymal masses. small calcifications. small cavities that communicate with

collecting system.

CT CT is the most sensitive modality for visualising renal

calcifications and CT IVP is more sensitive at identifying all manifestations of renal tuberculosis . 

early› papillary necrosis (single or multiple) resulting in uneven

caliectasis  progressive

› multifocal strictures can affect any part of the collecting system› generalised or focal hydronephrosis› mural thickening and enhancement› poorly enhancing renal parenchyma, either due to direct

involvement or due to hydronephrosis  endstage

› progressive hydronephrosis results in very thin parenchyma, mimicking multiple thin walled cysts

› amorphous dystrophic calcification eventually involves the entire kidney (known as putty kidney)

ctRenal tuberculosis. Contrast enhanced nephrographic phase CT shows dilatedcalices and thining of the renal cortex with thin calcifications.

Treatment and prognosis

Multi-drug treatment is essential, however despite treatment, stricturing can progress.

The role of nephrectomy is controversial and depends on the degree of renal impairment, bilateral vs unilateral disease and the status of the lower urinary tract. 

Nephrectomy, partial nephrectomy or cavernostomy can be performed both open and endoscopically 

Differential diagnosis

General imaging differential considerations include:

papillary necrosis medullary sponge kidney TCC (transitional cell carcinoma) of renal tract SCC (squamous cell carcinoma) of renal tract xanthogranulomatous pyelonephritis (XGP)

Emphysematous pyelitis Defined as isolated gas production

inside the excretory system, secondary to acute bacterial infection

It is relatively benign entity, and needs accurate differentiation from emphysematous pyelonephritis, which is much morbid condition.

It has excellent prognosis with good response to medical management .

Epidemiology and clinical presentation

Disease is more common in female, associated with diabetes and urinary tract

obstruction. Symptoms are usually mild, with fever and

chills. Sometimes flank tenderness with dysuria is present.

Aetiology E. coli, Klebsiella, Aerobacter aerogenes

and Proteus mirabilis are the commonest causative organisms.

Radiographic features

Plain film Retroperitoneal gas is seen in many patients.

Associated urolithiasis, if any, may also be seen.

Ultrasound May show dirty shadowing in pelvicalyceal

system. CT Gas-fluid level in dilated calyx, usually with

mild or no obvious features of pyelonephritis.

Plain film

ct

Treatment and prognosis

Emphysematous pyelitis genrally carries a good prognosis, and usually respond to intravenous antibiotics followed by oral antibiotics .

Differential diagnosis General considerations include emphysematous pyelonephritis reflux from emphysematous cystitis ileal ureterosigmoidostomy iatrogenic (ureteral instrumentation, radiological

intervention) fistulous connection with hollow viscus

Emphysematous pyelonephritis (EPN)

A morbid infection of kidneys, with characteristic gas formation within or around the kidneys. If not treated early, it may lead to fulminant sepsis and carries a high mortality.

Clinical presentation Flank pain, urinary tract obstruction with

fever. Leukocytosis and hyperglycemia (in diabetics) are prominent lab findings. Thrombocytopenia is particularly associated with poor prognosis 3.

Aetiology

It tends to be more common in females. Approximately 90% of patients have uncontrolled diabetes 1. It may however also be seen in immunocompromised individuals or associated with urolithiasis, neoplasms or sloughing of papilla.

Causative organisms include E. Coli: usually considered the commonest

causative organism 3

Klebsiella pneumonia Proteus mirabilis

Radiographic features

Plain film and fluoroscopy (IVP) May show mottled gas within renal

fossa or crescentic gas collection within Gerota's fascia. Linear gas shadows along paraspinal region may also be seen, representing retroperitoneal air.

Plain film

Ultrasound

may show an enlarged kidney with coarse echoes within renal parenchyma or collecting system.

dirty echogenic foci with reverberation/ring-down artifacts representing air ('dirty shadowing') may also be seen

Bowel gas over kidneys may lead to false positive diagnosis.

USG

CT

CT is the best diagnostic modality for emphysematous pyelonephritis. It may show following diagnostic features

enlarged, destructed renal parenchyma small bubbly or linear streaks of gas 1

fluid collections, with gas-fluid levels focal necrotic areas +/- abscess

CT

Radiological classification

CT features of emphysematous pyelonephritis differentiates into two types:

type 1› renal parenchymal destruction with streaky or mottled

appearance of gas› intra- or extrarenal fluid collections are characteristically

absent› it is usually more aggressive and lead to death shortly, if

not intervened early type 2

› renal or extrarenal collections associated with bubbly or loculated gas, or gas within pelvicalyceal system or ureter

The huang-tseng CT classification system   Class 1: gas in collecting system only Class 2: gas in renal parenchyma only (without

extrarenal extension) Class 3: gas in renal parenchyma with extrarenal

extension› Class 3a: extension of gas or abscess to perinephric

space› Class 3b: extension of gas or abscess to pararenal space

Class 4: bilateral emphysematous pyelonephritis or solitary kidney with emphysematous pyelonephritis

Treatment and prognosis In mild cases, intravenous antibiotics

are administered and percutaneous catheter drainage of perirenal or retroperitoneal collections is done. Severe cases often warrant a nephrectomy.

Differential diagnosis General imaging differential

considerations include emphysematous pyelitis iatrogenic (instrumentation, or

intervention of urinary tract) ureter-ileosigmoidostomy or fistulous

communication with bowel

MALAKOPLAKIA Malacoplakia of the urinary system is the

commonest manifestation of malakoplakia. The latter, meaning soft plaque, is a rare chronic granulomatous condition that can affect any organ.

Epidemiology Malacoplakia has a peak incidence in middle

age, and has a female-to-male ratio of 4:11. The disease is more common in patients who are immunocompromised or those with diabetes mellitus. 

Clinical presentation

Presenting symptoms include gross hematuria and signs of urinary tract infection, with Escherichia coli infection being very commonly found to coexist.  

Pathology Although infection with E. Coli is very commonly observed,

and it is thought to play a part in the pathogenesis of malacoplakia, other factors are also suspected, particularly impaired host defenses and defective phagocytosis 1.

The histologic hallmark of the disorder is the presence of basophilic inclusions, caIledMichaelis-Gutmann bodies in large eosinophilic macrophage2.

Location Within the urinary tract, the bladder is the most frequently

affected organ (40% of patients with malacoplakia)

Radiographic features

Imaging characteristics of malacoplakia are varied, and most commonly observed within the bladder, although plaques may also occur in the ureters.

Malacoplakia may present as multiple, polypoid, vascular, solid masses or as circumferential wall thickening, associated with vesicoureteral reflux and dilatation of the upper urinary tract. These masses vary in size from a few millimeters to several centimeters. 

Malacoplakia.

(a, b) Unenhanced CT images obtained at different levels (a higher than b) demonstratesymmetrically enlarged kidneys.

Malacoplakia

Corresponding T1-weighted MR image shows globally enlarged kidneys of intermediatesignal intensity. (d) Photograph of the bisected specimen reveals a whitish infiltrate that nearly completelyreplaces the renal parenchyma.

Treatment and prognosis

Although malacoplakia may be extremely aggressive, invading the perivesical space, and it can even cause bone destruction, non-surgical medical management is the mainstay of treatment, and as such biopsy for accurate diagnosis is essential. 

Treatment regimens include antibiotics, ascorbic acid, and a cholinergic agonist 1.

fungal pyelonephritis

The urinary system can be colonised by Candida Albicans either by the ascending or haematogenous route in immune suppressed patients.

Based on the clinical presentation and laboratory results, it is sometimes difficult to distinguish a colonisation from an infection.

However, the presence of Candida is always pathological. If it is present this points to systemic candidiasis and renal involvement is likely where it is also present in the urine.

Sonography detects accumulation of mycelia with a variable appearance: hyperechoic, minimally echoic or slightly echoic.

This is also visible on IVU or pyelography. Lesions of the renal parenchyma do not show any specific features.

On CT scanning, the mycelial accumulation does not take up the contrast medium. It has a rolled appearance when it contains gas between the layers of fungal colonies. When gas is absent, it appears as an aspecific but mobile solid mass. On an excretory phase CT scan, a filling defect in the collecting system can be seen.

Xanthogranulomatous pyelonephritis (XGP)

A rare form of chronic pyelonephritisand represents a chronic granulomatous disease resulting in a non-functioning kidney. Radiographic features are usually specific.

Epidemiology Xanthogranulomatous pyelonephritis is seen

essentially in all age groups, but most frequently presents in middle-aged to elderly patients .

There is a female predilection (F:M 2:1) presumably relating to an increased incidence of urinary tract infections and thus struvite (staghorn) calculi.

Clinical presentation

Clinical presentation is typically vague, consisting of constitutional symptoms such as

Malaise, Weight loss and Low grade fever. Haematuria and flank pain are sometimes

encountered . Despite often absent urinary tract symptoms,

pyuria and positive urinary cultures are present in the majority of cases (95 and 60% respectively) .

Pathology Xanthogranulomatous pyelonephritis is, as the name

suggests, a chronic granulomatous process believed to be the result of subacute/chronic infection inciting a chronic but incomplete immune reaction .

The kidney is eventually replaced by a mass of reactive tissue, surrounding the usually present (90%) inciting  staghorn calculus with associated hydronephrosis of a greater or lesser degree. Foamy (lipid laden) macrophages predominate .

Inflammatory process eventually extends into perinephric tissues and even adjacent organs .

Various bacteria are isolated, however the most commonly isolated species are E coli andP mirabilis .

Radiographic features

Two forms of the disease are recognised both macroscopically and on imaging :

diffuse: 90% of cases focal: 10% of cases

› sometimes truly a focal process in a normal kidney

› in other instances this represents diffuse XGP of one moiety of a duplex system

Radiographic features Plain film Plain film findings are difficult to

distinguish from a routine staghorn calculus, although fragmentation and enlargement of the the renal outline may be seen. A calculus is not always present; in such cases it is not possible to make a plain film diagnosis.

Plain film

Ultrasound

Ultrasound examination demonstrate an enlarged and distorted renal outline, with loss of the normal renal architecture and (usually) a centrally located shadowing calculus.

Ultrasound

CT

CT findings are most helpful in reaching the correct diagnosis. The normal renal outline is lost and enlarged with paradoxical contracted renal pelvis. The calyces in contrast, are dilated giving a multloculated appearance that has been likened to the paw print of a bear (bear's paw sign) . Sometimes there is perinephric extension with thickening of the Gerota's fascia. Calcification can be better delineated on CT scan. 

CT or conventional urography In most cases there is little if any renal function in

the affected kidney .

CT

MRI MRI appearances mirror the

heterogeneous nature of the mass with solid and cystic components surrounding a central staghorn calculus. As such signal is heterogeneous on all sequences.

MRI

Treatment and prognosis

By the time xanthogranulomatous pyelonephritis has become established, no conservative or medical therapies exist. Surgical nephrectomy is usually curative . The presence of inflammatory reaction in adjacent tissues often requires a large operative field and an anterolateral transperitoneal approach .

Differential diagnosis

The differential is narrow when the entire kidney is affected and cross-sectional imaging has been obtained, and is largely limited to renal tuberculosis, which however usually results in a shrunken calcified putty kidney.

In cases where typical features are not present (e.g. no staghorn calculus, focal disease only) then other entities to be considered include:

renal tuberculosis renal abscess renal cell carcinoma (RCC) angiomyolipoma (AML): with minimal fat

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